547

The Canalian M ineralo g ist Vol.36, pp. 54'1-561(1998)

COMPOSITIONAL,STRUCTURAL AND PHASE RELATIONSHIPSIN TITANIAN IXIOLITEAND TITANIAN COLUMBITE.

PETR EERNfI

Depanment of Geological Sciences, University of Manitoba, Wnnipeg, Manitoba R3T 2N2

T. SCOTTERCIT

Research Division, Canadian Museum of Nature, Onawa, Ontario KI P 6P4

MICHAELA. WISE

Department of Mineral Sciences,Smithsonian Institution, Washington,D.C.20560, U.S.A.

RON CHAPMAN ANDHARVEY M. BUCK

Departtnent of Geological Sciences, University of Manitoba, Wnnipeg, Manitoba R3T 2N2

ABSTRACT

Titanium-enriched members of the columbite family (orthorhombic) commonly contain exsolved niobian-tantalian , or are aggregatedwith a coprecipitated rutile phase.This assemblageis the Fe,Mn,M,Ta-rich counterpart of the Ti-dominant niobian rutile + exsolved titrnian columbite pair. The rutile phase is enriched in Fe2*,T4 Fek and Sn, whereasthe orthorhombic phase favors Mn, M, Sc, W and Zr. Most samples of the orthorhombic phase are defined as titanian columbite - tantalite; they have low to moderate Ti content, moderately to considerably disordered structure, and convert to highly ordered columbite - tantalite on heating. A minor number of samples of the orthorhombic phase develops the highly ordered structure of on heating, which identifies them as titanian ixiolite, the disordered counterpart of titanowodginite. These samples are highly disordered in the natural state.They are Ti-rich, with Mn > Fe and Ta > Nb; these features correspond to the composition of natural titanowodginite and of the only titanowodginite synthesizedto date. However, subordinatebut significant Sn seems to be present in all samples of titanian ixiolite as well as natural titanowodginite, and it may affect the path of ordering. Excess of Ta over the (Fe, MnXNb, Ta), stoichiometry of columbite, characteristic of most compositions of wodginite, may play a significant role. Ordered nuclei potentially present in the bulk of natural disordered phasesalso may be important.

Keryords: , columbite, tantalite, ixiolite, order-disorder, exsolution.

Souuene

l,es memb'resdu groupe de la columbite (orthorhombique) qui sont enrichis en titane possbdentg6ndralement des domaines de rutile enrichi en Nb et Ta dus tr l'exsolution, ou bien des agr6gatsde rutile copr6cipit6. Il s'agit de I'assemblage contenant Fe, Mn, M et Ta qui est la contrepartie de I'association rutile niobifere + columbite titanifBre exsolv6e. La phase rutile est enrichie psz*, Ta, Feh et Sn, tandis que la phase orthorhombique favorise Mn, Nb, Sc, W et Zr. Dans la plupart des cas, la phase orthorhombique"o fait partie de la s6rie columbite - tantalite titanifdre; elle possbdeune teneur faible d mod6r6e en Ti et une structure relativement d6sordonn6e,et se transforme en columbite - tantalite bien ordonnde en chauffant. Dans un nombre restreint de cas, la phase orthorhombique se transforme en wodginite bien ordonn6e en chauffant, ce qui les identifie comme ixiolite titanifdre, la contrepartie d6sordonn6e de la titanowodginite. Ces 6chantillons sont foftement d6sordonn6s i l'6tat natuel. Ils sont riches en Ti, avec Mn > Fe et Ta > Nb; ces caract6ristiquescorrespondent h la composition de la titanowodginite naturelle et du seul exemple qui en ait 6t6 synthdtis6jusqu'd maintenant.Toutefois, il semble que des quantitds non n6gligeables d'6tain soient pr6sentesdans tous les 6chantillons d'ixiotite titanifdre et de titanowodginite naturelle, et ceci pourrait influencer la mise en ordre. Un exc6dent de Ta par rapport I la stoechiom6trie Fe,MnXNb,Ta)2 de la columbite, qui caract6risela plupart des compositions de wodginite, pourrait aussi jouer un 16le important. Les domaines ordonn6s potentiellement pr6sents dans les phasesnaturelles d6sordonn6espourraient aussi €tre importants. (fraduit par la R6daction)

Mots-cI6s: titane, columbite, tantalite, ixiolite, ordre-d6sordre, exsolution.

t E-mail address: [email protected] 548

INTRoDUCTIoN Unit-cell dimensions are provided for the natural phase by Sveshnikova et at. (1965) and dernf er aI. (1979), Ixiolite was redefinedby Nickel et al. (1963) as a and are complemented by data on heated material by complex oxide of the @,B)O, type, a Sn-substituted, Paul (1984), Ferreira (1984) and Cern! et at. (1989). cation-disorderedderivative of the 4820.6columbite The above sources suggest that the Ti-enriched structure (in which A = Fe, Mn, Mg, and B = Nb, Ta). orthorhombic minerals of the ABrOuor (A,B)O, types More specifically, ixiolite was assignedthe composition are either homogeneous,or closely associatedwith (Ta,Nb,Sn,Fe,Mn)Or,with an orthorhombic unit-cell a rutile phase. They show variable degrees of cation corresponding to that of disordered columbite; on disorder, and attain on heating either an ordered heating, it was reported to transform into a presumably columbite structure or a wodginite structure. However, orthorhombic "olovotantalite'Llike phase. the data are too limited for any attempts to quantify and "Olovotantalite" (Matias 196l) was later identified generalizethese relationships and their controls. with monoclinic wodginite (Gorzhevskayaet al. 1974), and further study of samplesof Sn-rich ixiolite from Savpr-ns Exavwep numerous localities established its ordering upon heating to wodginite as a rypical feature flilise & eernf Table I provides condensedinformation on the 1986,Wise 1987).The presenceof a slight monoclinic sources ofdatafrom the literature, and on the samples distortion in samples of natural (pseudo)orthorhombic examined during our current work. The table also ixiolite, indicative ofincipient ordering, was detected presentsthe acronyms used to designatelocalities in by the above authors, apparently converging with most of the figures and some of the tables below. In highly disordered pseudo-orthorhombic wodginite total, data on titanian columbite - tantalite and ixiolite fl.;ahn D82, Wise 1987, Ercit 1986, Ercit et aI. t992a). from four localities were exffacted from the literature, Thus, stannianixiolite is establishedas a disordered besides those from five localities examined in our counterpart of the ordered wodginite. laboratories and published over the last 17 years. Compositional variations led to recognition of Some of these data were in part complemented during additional varieties of ixiolite. namelv titanian ixiolite. the current work, which covered samples from 11 scandian ixiolite and "wolframo-ixiolite" (iernf & additional localities. Ercit 1985, 1989, Wise & Cernf 1986). Whereas the A broad compositional range is covered by the crystal-chemical and structural behavior of scandian selected samples, with the TiO, content of the ixiolite and "wolframo-ixiolite" remained essentially orthorhombic phase variable from -2 to -20 wt.Vo. uncharted, titanian ixiolite was generally considered a Both homogeneousminerals and exsolved intergrowths close analog of its Sn-rich countepart(e.5., eemg & with rutile are represented, as well as orthorhombic Ercit 1985, 1989). However, significant differences phases associated with rutile in a texturally non- were subsequentlyfound in the responseof Ti-rich replacive and unexsolved, but side-by-side coexisting samplesto heating; widespread presenceof associated manner. In some cases,the orthorhombic phase and rutile was noticed, and the need for a systematic study rutile are dispersedin the parent rock, but noi in mutual was recognized. contact; in others, rutile is absent. Here we present a review of data from the literature Care was taken to select samples of reasonably and our new observations,as a first step toward proper simple compositions,free of substantialquantities of characterization and definition of titanian derivatives other components that may influence the structural of the columbite - tantalite and ixiolite minerals, and behavior or phase relationships of the orthorhombic of their relation to the wodginite and rutile groups of mineral on heating. In this respect,the most significant minerals in the system TiO2 - (Fe,Mn)z*(M,Ta)s*rOu- element is Sn, which also is the most common one in Fern(Nb,Ta)Oo. the minerals examined. Only samples that show Ti/Sn (at.) greater than 2 were used here; unfortunately, PnrvlousWom Ti-rich specimens truly poor in Sn are quite rare. Scandian ixiolite, notorious for its high Ti content, was Information on the Ti-enriched members of the not employed. family of columbite minerals sensu lato (ordered to disordered columbite group, ixiolite, wodginite group) Expennmrrnr Mnrgops is so far rather meager, dispersed in papers describing individual occrurences,and is in most casesincomplete. Chemical composition was establishedby means Data on chemical composition are commonly not of electron-microprobeanalysis. Some of the data were accompaniedby structural characteristics(eemj et al. obtained using the MAC-5 instrument in energy- 1981a, Zagorskyi & Peretyazhko 1992, Johan & Johan dispersion mode, under conditions given in Cem! et al. 1994); this is due in part to a fine grain-size of the (1986). The other data were generatedin wavelength- samplesand dispersal in and intimate intercrowth with dispersion mode on the CAMECA SX-50 apparatus" othei phases((em! et al. 1986, dernf & NEmec 1995). using the sameanalytical conditions and standardization TITANIAN IXIOLITE AND TITANIAN COLUMBITE - TANTALITE 549

TABLE 1. LIST OF EXAMINED SAMPLES Locality Comp.# Associationr Source/Reference2 CentralTransbaikalia, Russia ZAG-I, ta -2 hom"nor Zagorskyi& Peretyazhko(1992) CfnovecoCzech Republic JOH-3to 4 hom,sep Johan& Johan(1994) Tanco,Manitoba TRT-5 to -29 coex researchcoll. PC VEZn6IIst, CzechRepublic VZS-30to -31 coex tem! et al. (1,979) Siberiaand Urals, Russia SVE-32to -33 hom"nor Sveshnikovaet al. (1965) North Star"Texas. USA NST-34to -66 hom & exs researchcoll. Pd Weinebene,Austria BRD-67to -94 exs res.coll. Pd; demf et al. (1989) Un-known NN-95 to -96 coex researchcoll. Pi CanyonCity. Colorado"USA CCC-97ro -98 exs usNM #t24389 La Modera,New Mexico,USA LMN-99to -100 exs usNM #1021't8 Guffey"Colorado, USA GUC-101ro -102 exs USNM #96418 Norway NOR-103to -1M exs usNM # 107397 Ambatofotsikely, Madagascar AMB-105to 1020 exs usNM #107397 HuronClaim, Manitoba IIIJC-121. to -124 hom,sep PauI(1984) MuskoxLake, NWT MSX-125to -136 exs researchcoll. Pi Greer Lake, Manitoba GL-137to -140 coex tern! et al. (1986) VdZndII, CzechRepublic KVV-141to -154 exs researchcoll. Pd K-1, Madtoba K-1-188to -183 exs researchcoll. Pd Bradlo,Czech Republic JIH-185to -186 hom,sep iernf & NEmec(1995) Lower Tanco,Manitoba LOT-I87 to -226 exs,hom Ferreira(1984) and this study Irgon Claim, Manitoba IRA-227 ta -229 coex researchcoll. Pd

t hom - homogeneousorthorhombic phase; exs - onhorhombicphase with exsolvedrutile; coex- orthorhombicphase with coprecipitatedrutile in mufual contact;sep - dispersedseparate gfains of orthorhombicphase and rutile; nor - no rutile associatedwith the orthorhombicphase. 'Soutces quotedfor samplesinvestigated during the presentstudy, references given for data taken from the literature.

as Uher et al. (1998), except that a 50 s counting X-ray powder-diffraction data were collected on time for minor elements was used, and ThO2 glass for two powder diffractometers. A Philips PW 1710 ThMa. Formula contents were calculated on the instrument was used with experimental conditions basis of 24 atoms of oxygen and 12 cations per unit and processing as reported in Cernf et al. (1986). cell of ordered columbite - tantalite. This might seem A Siemens D 5000 diffractometer was employed misleading for bot[r the disordered orthorhombic recently in transmissionmode, with a primary beam phases(8 atoms of oxygen per unit cell) and rutile monochromator (CuKa1) and calibration with annealed (4 atoms of oxygen per unit cell), but it facilitates direct CaF2 internal standard, a = 5.465397(4) A. Unit-cell comparison of cation abundances and proportions dimensionswere calculatedwith the program CELREF in all minerals analyzed in this srudy. Apparently (Appleman & Evans 1973). cation-excessresults were accommodatedby converting as much of Fez* to Fe! as required to satisfy the second Prnsr CovrposmoN limitation above; for the legitimacy of this procedurein minerals of the columbite familv. see Ercit et al. As shown in Table I, Ti-rich orthorhombic phasesat (r992a). most of the localities examined contain exsolved 550

FIc. 1. Tlpical textural relationships of the orthorhombic and rutile phases in back-scattered electron (BSE) images. A. Exsolution-induced droplets of niobian rutile (black) and (white) i.n slightly heterogeneousorthorhombic phase (monled grey), Weinebene,Austria. B. Exsolution-induced blebs of niobian rutile (black) and droplets of cassiterite (white) in near-homogeneousorthorhombic phase (pale grey), Muskox Lake, N.W.T. C. Dendritic to mossy segregationsof niobian rutile (black to dark grey) in an orthorhombic phase (grey), VdZn6, Czech Republic. D. Joint-related exsolution of rutile (dark shades of grey) and cassiterite (white) in an ofthorhombic phase (pale grey), K-I, Manitoba. E. Fine-grained exsolution-induced domains of tantalian rutile (dark grey) in an orthorhombic phase (pale grey), but coarsening along contacts with inclusions of (white), Lower Tanco, Manitoba. F. Heterogeneous exsolution-induced domains of tantalian rutile (mottled dark grey), in part associated with a microfracture, in an orthorhombic phase (pale grey) with a few inclusions of microlite (white), Lower Tanco, Manitoba. Scale bars are 100 prn long, except 50 pm for F. TITANIAN IXIOLTTE AND TITANIAN COLTJMBITE - TANTALITE 551 niobian or tantalian rutile. A small number of occur- The orthorhombic phase rences shows these two phases in mutual contact, but seemingly not exsolved.At some localities, the rutile The composition of the orthorhombic phase seems phaseis either absentor not related to the orthorhombic to cover about the same range in both homogeneous mineral. minerals and those associatedwith rutile (Figs. 3A, B). Examples of typical exsolution-induced textures are In the latter case,M- and Fe-rich compositions se€mto shown in Figure l. Blebby, amoeboid outlines of the be preferred in many samples,but not to the exclusion rutile phase,the density of its distribution inversely of Ta- and Mn-rich phases(Figs. 3A, B). proportional to its grain size, and local crystallographic Orthorhombic minerals paired with rutile extend control on the distribution and shape of its grains, all over a slightly broader range of Ti (+Sn) contents strongly indicate an exsolution origin. This conclusion than the rutile-free phases (Figs. 3C, D). A significant is also supported by the commonly Iow Ti content proportion of the data fall above the (Ti + Sn) - of the orthorhombic phase in areascontaining dense (Fe,Mn)2*(Nb,Ta)s*,tie line, indicating a surplus of accumulationsor relatively large grains of rutile, in (Nb,Ta) over the stoichiometry of columbite. contrast to neighboring patches of homogeneous Besides the main components (MzOs, Ta2O5,FeO, orthorhombic phase, which are routinely Ti-rich. FerO3,MnO, TiOt, the orthorhombic phasecommonly However, primary inclusions of rutile are suspectedin contains subordinateSnO2 (<6.65 wt.Vo,but in most casesof compositionally homogeneousorthorhombic casesin the 0.X--2 wt.Vorange), minor ZrO2 (<1.30 host. wt.Va),WO3(<3.88 wt.Vo), CaO and MgO [<1.15and At some localities, granular aggregates of the 31.41 wt.Vorespectively, as reported by Sveshnikova et orthorhombic phase show no textural evidence of aI. (1965), but negligible in most casesl, and Sc2O3 exsolution, and no features suggestive of replacement. (32.02 wt.Vo).The contentsof UO2,ThOz, BizOg,and In their present state, they represent products of PbO are very low and in most casesbelow detection more-or-less simultaneous coprecipitation with the limits. As could be expected, the abundancesof minor rutile phase. elements show remarkable differences among indi- The ZAG and SVE samples of homogeneous vidual localities, controlled by the overall geochemistry orthorhombic minerals are the only ones that are of the parent mineral assemblages. probably totally free of any form of associatedrutile. Besides the homovalent substitutionsin theA and B A part of the NST samplesalso is homogeneous,but sites (Fe, Mn, Mg, Ca and Nb, Ta, respectively), which most of its grains are exsolved.Some of the LOT crystals are common to all members of the columbite family, are homogeneousand not accompaniedby rutile of any the heterovalent mechanism introducing Fe$ and most kind. The JOH phasesare associatedwith niobian rutile probably also Sc3*is widespread:@e,Sc):*rNbs**'Fez1. dispersed in a granitic rock, but are not in mutual The entry of Ti and Sn is facilitated mainly by contact. The same relationship applies to the JIH and (Ti,Sn)&3(Fe,Mn)2*-1(Nb,Ta)s+-2and possibly also by HUC pegmarites,in which rutile is very scarce. (Ti,Sn)6rFe3* r(Nb,Ta)*-r . In the JOH samples,which contain appreciableW, this element is incorporated vla Cunurcar- CorwosmoN the substitution WG(Ti,Sn)(M,Ta)5* 2 (Johan & Johan 1994). The contents of the other minor elements are AII compositions obtained during the present study too low to decipher their mode(s) of substitution, as and extracted from the literature are available from the the relevant calculations operate close to the limits of Depository of Unpublished Data, CISTI, National analy-tical error. ResearchCouncil of Canada, Ottawa, Ontario K1A 0S2. Table 2 shows selected representative data. The Rutile diagrams used here do not show all data as collected in the deposited table; numbers of compositions were The rutile phase shows extensive solid-solution reduced for some samples to minimize locality bias. of both the columbite - tantalite and Fe3*(M,Ta)Oa Figure 2 illustrates the composition ofall analyzed, components (Fig. 2). In addition to the dominant minerals in the columbite quadrilateral, and in the diagram components(TiO2.Nb2O5, Ta2O5, FeO, Fe2O3),rutile (Ti + Sn) - (Ta + Nb) - (Mn + Fe). In the quadrilateral, routinely contains minor MnO (<0.95 w t.Va)and SnO2 the rutile compositions are restricted to the Mn-poor (32.40 wt.Va). The contents of WO3 (

TABLE 2. REPRESENTATIVECOMPOSIIONS OF-THE ORTHORHOMBICPTIASE AND ASSOCIATEDRIITILE

K-l Muskox L. Norih Sta Vdm6 II Weinebene Ambarofotsikely Lower Tmco Tmco r57X l58R t32X t33R 62X 6tR t49X l45R 80X 79R r20X lrgR 22lX 2Z3R- r6X r5R wo. t.ig 0.00 t.20 0.22 L36 0.00 2.02 0.20 l.r6 0.16 0.l4 0.00 0.00 0.00 0.00 0.00

TqO, 24.23 13.42 9.84 8.55 I 1.86 r l.3l 1t.67 t1.29 LJ.CL IJ.IJ 5.20 6.51 63.10 27.44 51.14 27.40 Nb,o! 50.39 7.64 65.59 10.75 61.82 10.35 59.37 16.75 60.35 17.50 68.05 t9.3E 9.53 1.50 26.92 2.26 uo, 0.28 0.00 0.00 0.00 0.3't 0.00 0.r9 0.00 0.29 0.00 0.00 0.00 0.00 0.00 0.43 0.00 Zfr, 0.27 0.00 0.00 0.00 0.23 0.00 0.E5 0.00 0.54 0.00 0.59 0.00 0.00 0.00 0.46 0.00 SnO, 0.61 0.30 0.15 t.08 0.06 0.69 1.08 2.82 0.90 3.18 0.41 1.78 3.68 0.92 0.92 l.l4 Tio, 2.E8 69.85 2.29 74.14 2.94 67.60 6.64 59.67 4.63 57.19 5.22 59.85 10.45 64.32 3.53 62.34 Bi,O! 0.18 0.00 0.00 0.00 0.00 0.00 0.14 0.00 0.00 0.00 0.00 0.12 0.00 0.14 0.00 0.00 sb,03 0.00 0.00 0.00 0.00 0.08 0.00 0.00 0.29 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yr0, 0.00 0.00 0.00 0.00 o.z2 0.00 0.00 0.00 0.00 0.00 0.23 0.00 0.00 0.00 0.00 0.00 SqO, 0.00 0.00 0.22 0.00 0.00 0.00 0.85 0.08 0.28 0.00 1.70 0.04 0.4't 0.00 0.69 0.00 Fe,O, 1.95 6.99 1.39 3.21 z.to 7.82 0.90 4.92 0.43 I.58 r.06 10.64 0.00 0.8E 0.09 0.l9 FeO 8.81 l.OZ 9.7s 2.79 2.30 0.80 13.15 3.41 t2.13 6.t6 9.96 1.29 r.90 4.33 3.16 4.79 MnO 7.77 0.08 9.07 0.09 15.45 0.31 t.43 0.06 6.15 0.14 7.49 0.18 9.9r 0.12 I 1.88 0.17 CaO 0.20 0.00 0.02 0.03 0.06 0.00 0.03 0.28 0.00 0.00 0.00 0.00 0.o2 0.00 0.00 0.00 Meo 0.05 0.00 0.00 0.00 0.00 0.00 1.72 0.t5 0.09 0.00 0.1'1 0.00 0.00 0.00 0.00 0.01 total 98.80 99.30 99.52 100.E6 98.85 98.88 100.04 99.92 100.35 99.64 t00.22 99.79 99.06 99.65 99.72 98.30

w6, 0.076 0.000 0.072 0.0r0 0.083 0.000 0.1r9 0.010 0.070 0.008 0.008 0.000 0.000 0.000 0.000 0.000 T^5. 1.646 0.661 0.623 0.409 0.762 0.562 0.723 0.576 0.851 0.7tE 0.315 0.324 4.965 1.461 3.900 1.487 Nb5' 5.692 0.629 6.907 0.854 6.607 0.E55 6.n4 1.420 6.363 1.521 6.847 1.604 t.247 0.133 3.4t3 0.2M 0.016 0.000 0.000 0.000 0.019 0.000 0.010 0.000 0.0r5 0.000 0.000 0.000 0.000 0.000 0.027 0.000 zl' 0.033 0.000 0.000 0.000 0.027 0.000 0.094 0.000 0.061 0.000 0.064 0.000 0.000 0.000 0.063 0.000 Sno' 0.061 0.022 0.014 0.076 0.006 0.050 0.098 0.2u 0.084 0.244 0.036 0.r 30 0.425 0.072 0.103 0.091 0.541 9.560 0.401 9.79E 0.523 9.281 l.l3'7 8.4t2 0.8t2 8.268 0.874 8.239 2.274 9.469 0.144 9.358 Bf' 0.012 0.000 0.000 0.000 0.000 0.000 0.00E 0.000 0.000 0.000 0.000 0.006 0.000 0.007 0.000 0.000 SbJ, 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.022 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Y3. 0.000 0.000 0.000 0.000 0.028 0.000 0.000 0.000 0.000 0.000 0.027 0.000 0.000 0.000 0.000 0.000 sc' 0.000 0.000 0.045 0.000 0.000 0.000 0.169 0.013 0.057 0.000 0.330 0.006 0.1l9 0.000 0.169 0.000 fe' 0.367 0.957 0.244 0.425 0.374 t.075 0.155 0.695 0.075 0.2'28 0.t77 t.465 0.000 0.t30 0.0r9 0.028 l-e' 1.840 0.156 1.E99 0.410 0.4s5 0.t23 2.505 0.535 2.365 0.e90 1.854 0.19E 0.460 0.708 0.741 0.800 Mn" t.644 0.012 1.790 0.0r3 3.094 0.048 0.276 0.010 t.2t5 0.023 1.4t2 0.028 2.429 0.020 2.822 0.029 0.054 0.000 0.005 0.006 0.015 0.000 0.007 0.056 0.000 0.000 0.000 0.000 0.006 0.000 0.000 0.000 0.019 0.000 0.000 0.000 0.000 0.000 0.s84 0.M2 0.031 0.000 0.056 0.000 0.000 0.000 0.000 0.003

X - orthorhombicphase,R - rutile Atomic contenr baed on 24 oxygen atoms and 12 cations of the ordercd columbite ell, to facilitate mutual compaison; FerO. calculated to eliruale etion surplu over 12.000 (cJ. Ercit et al. 1992a).

Scarcity and low concentration of the minor elements and Nb, and the affinity of Fe and Ta for rutile, are do not permit meaningful interpretation of their entry expressedin all mineral pafusexamined, disregarding into the structure of the rutile examined. what may be the specific values of Mn/(Mn + Fe) and Tal(Ta + Nb) of the minerals, the exsolved or E lement p a rt it ion in g b etw een the o rt horhomb ic coprecipitated nature of their association,their relative phase and rutile abundances, or their provenance (Fig. 4). Strong preferences also are shown by the distribution of The orthorhombic-phase and rutile populations are subordinate and minor components (Table 2). The separatedby their Ti contents (Fig. 2B), and by the orthorhombic phase invariably concentrates WO3, strong tendency of rutile to exclude Mn (Figs. 2A, 4). Sc2O3and ZrOr, commonly to their virtual exclusion The preference of the orthorhombic phase for Mn from rutile. In contrast, the SnO, and FerO, contents of TITANI-AN IXIOLITE AND TITANIAN COLUMBITE _ TANTAIITE 553

o D A o lv ,t !t

ll v tt t zo.e ? Y rt + tl fr (U ro t f^" ' F D- b"oo t fo ' 'Ylr -l\ rltt v J' I,;tt, Y F -t! ottt-. ? 0.0 L t00 0.0 0..1 0.0 1.0 020 &) lm Mr/(Mn+ Fe) Ti+Sn Mn+Fe

Hc. 2. Summary of all data collected for this study, expressedin terms of atomic proportions in the columbite quadrilateral (A), and in the triangular plot (Ta + Nb) - (Ii + Sn) - Q4n + Fe) [B). Triangles: the orthorhombic phase, circles: rutile.

rutile are considerably higher than those of the contents (e.g., the NST compositions43-49 and 53-58, orthorhombic phase,with only a few exceptions. 60,62, BRD 67-74,CCC, LMN and GUC97,99 and Figure 5 illustrates the compositions of coexisting 101, respectively).These phasesextend into the most orthorhombic and rutile phasesat individual localities. disordered region of the diagram, which also contains It is evident that the extent of solid solution of TiO2 in the Ti-rich samples(TRT 5-7, 16,LOT 187-191,221). the orthorhombic phase increases concomitantly with The only exceptions to the above distribution are the that of the (Fe>>Mn)2*(Nb,Ta)rO6 and Fe3*(Nb,Ta)Oo Ti-rich samplesexamined by Sveshnikova et al. (1965; componentsin the rutile phase.It also is evident that 32 and 33), which display a and c values indicative the solid solution is in all casesslightly to distinctly of a considerable to high degree of order. However, more extensive in the rutile phase than in the the magnitude of standard error, the age and the orthorhombic mineral. single-crystalmethod used in the above study do not encourage confidence in the quality of these data. Srnucrunar Srare The a - c plots are generally shifted from the region outlined by ordered and disordered columbite - The orthorhombic phase in natural state tantalite sensu stricto toward lower values of c and particularly a. This is in agreementwith the results of Unit-cell dimensions of samples representedin Ercit et al. (1995), who quantified such an effect of Table 2 are listed in Table 3. Figure 6 summarizes Ti substitution on the unit-cell dimensions of the the available data on unit-cell dimensions of the columbite - tantalite minerals. orthorhombic phasein the a versus c diagram of eernf & Turnock (1971), as progressively modified by Wlse et al. (1985) and Ercit er al. (1995). This diagram The orthorhombic phase after heating shows the variations induced by the Fe-Mn substitution (subparallel to a), and by the transition from ordered Heating promotes the ordering of cations in A-site (Fe,Mn) + B-site (M,Tal distriburion of carions columbite-type minerals, and standard corditions to disordered population of all cations in a single of 1000"C for 16 hours were adopted by Cern! & octahedral site (in diagonal direction). Turnock (1971) to facilitate mutual comparisons.These It is evident that the data cover almost the full span conditions are sufficient to effect virrually complete of structural states,except the region of highly ordered ordering in natural samples of diverse structural state, structures.Comparison with chemical compositions including those with total disorder of the cations. shows that most of the data in the lower half of the Heating of granular (-1-2 mm) material in air produces diagram, indicative of moderate to considerabledegree effects indistinguishable from those achieved in an of order, belong to phases with low to moderate Ti oxidation-suppressing atmosphereof CO + CO2. s54

homogeneous matrixof exsolved A B or associatedrutile

o -o t tv z0.6 z0.a Y + + (U (U F F Yo.a (U Yo.a(U F F

o.2 vlvl !tY V 7t, tt, t-t F V

0.0 0.0 L 0.0 0.4 0.6 1.0 0.0 0.4 0.6 Mr/(Mn+ Fe) Mn/(Mn+ Fe) Ta+15 Ta+Nb 01@ 0 1q) homogeneous matrix of exsolved or as.sociatedrutile 80 80

e,Mn12'1Nb,Ta;$, Ta)$z 60

Fe$1Nb,Ta;s Fe3*1Nb,Ta1e 40

1@2 0 60 80 100 o20 60 80 100 Ti+ Sn Mn+Fe Ti+5n Mn+Fe

Ftc. 3. Composition ofthe homogeneousorthorhombic phase (A, C) and ofthe orthorhombic phasewith exsolved or associated rutile @, D) in the columbite quadrilateral and in the (Ta + Nb) - (Ti + Sn) - (Mn + Fe) system (atomic proportions.,r.

Of the samples subjected to heating, those with Rutile low to moderate contents of Ti and low to moderate disorder were converted to more or less fully ordered Becauseof its subordinate quantities in exsolution- columbite - tantalite (Fig. 6). However, samples with induced intergrowths and aggregates with the high Ti (and Mn) content and high degree of disorder orthorhombic phase, rutile usually generatesonly a in the natural state (TRT 5, 6,7 , 16 and LOT 187-191, few i ndi vidual ized X-ray powder-diffraction maxima. 221, italicized in Figure 6) developed the wodginite Refinement of its unit-cell dimensions is not feasible in structure (Fig. 7, Table 3). most cases.However. diffraction maxima indicative of TITANIAN IXIOLITE AN'D TITANIAN COLIJMBITE - TANTALITE 555

0.8

-o z0.6 + (U F Yo.r(E LMN F K-1

N/V '-*t)r, vzs GUC ccc BRD 0.0 L 0.0 L 0.0 1.0 0.0

0.8 LOT --:':' ' 1\. .o t\. \\\' zo.6 t) + lRn (E NOR F Yo.l o F -"-i_\---

o.2 Q NST ---_, \\ V AMB KW 0.0L 0.0 L 0.0 1.0 0.0 0.4 0.6 0 8 Mn/(Mn+ Fe) Mn/(Mn+ Fe)

Ftc. 4. Compositions of exsolved or associatedpairs of the orthorhombic and rutile phasesin the columbite quadrilateral (atomic proponions). Symbols as in Figure 2.Locality acronyms as listed inTable 1.

a trirutile superstructure were never observed. Thus it is Dlscussrou possible to assumethat the structure correspondsto that of pure rutile, with disordered distribution of Phase relationships non-integral quantities of substituting elementsover octahedral positions representing a single cation site. In most samples examined, the Ti-enriched After heating, the diffraction maxima of rutile orthorhombic phase contains exsolved rutile; in many display about the same intensities as those generatedby others, the orthorhombic phaseis associatedwith rutile the natural phase.This findirg indicates that dissolution in aggregatessuggesting simultaneous crystallization. of rutile in the orthorhombic phase must have been In a few other instances,the orthorhombic phase is minimal. dispersed among rock-forming silicates along with 556 THE CANADIAN MINERALOGIST

Ta+Nb Ta+Nb Ta+Nb

Ti+Sn Mn+Fe Ti+Sn Mn+Ps Ti+Sn Mn+Fe

Ta+Nb Ta+Nb Ta+Nb

Ti+Sn Mn+Fe Ti+5n Mn+Fe Ti+Sn Mn+Fe

Ta+Nb Ta+Nb Ta+Nb 7 ,z

Ti+5n Mn+Fe Ti+Sn Mn+Fe Ti+Sn Mn+Fe

Flc. 5. Compositions of the orthorhombic phase and exsolved or associated rutile at individual localities, in (Ta + Nb) - (Ti + Sn) - (Mn + Fe) system. Symbols as in Figure 2,locality acronyms listed in Table l.

rutile, but not in mutual contact. Occurrences of a rutile, which commonly exsolves an orthorhombic homogeneous orthorhombic phase devoid of any phase(e.g., Lebedeva 1968, Sahama 1978,(ernf et al. association with rutile seem to be very rare. 1964, 1981b,1986). The difference between the two In many cases,the orthorhombic phase evidently casesis in the bulk composition and structure of the crystallized as a metastable,homogeneous, disordered homogeneousprecursor; the roles ofthe host mineral mineral with a tendencyto exsolve rutile on cooling, and the exsolved phase are reversed. and only rarely was it preserved in the originat In contrast to niobian-tantalian rutile, which homogeneous state. In this respect, the examined exsolvestitanian columbite if M-dominant but remains exsolution-inducedintergrowths are a counterpartof in most caseshomogeneous if its original composition those exhibited by niobian (and, to a degree,tantalian) is Ta-dominant (Cem! & Ercit 1989), there seemsto be TTTAMAN IXIOLNE AND TITANIAN COLUMBITE _ TANTAIITE 557

5.15

o C,A *ffi 5.to

o 14.30 orA

FIc. 6. The a - c plot of unit-cell dimensions for the orthorhombic phase,with a expressed in values corresponding to the ordered columbite superstructure to facilitate comparison. Data for end-member compositions from Ercit et al. (1995). Data for compositions that order to the wodginite structure upon heating are labeled in italics; other samples develop the ordered columbite superstructurewith unit-cell dimensions marked at the unnumbered ends of the tie lines. Crosses indicate standard deviations of 1o.

no such compositional control on the exsolution behavior of the titanium-rich orthorhombic phase: theZAG and homogeneousNST samples are Mn- and M-rich; the fwo SVE specimensare extremely M-rich and highly variable in Mn, and both apparently contain a remarkably high percentage ofFe+. In contrast, the homogeneous LOT samples are Mn- and Ta-rich, and the HUC samplesare Fe- and M-dominant (Tables2, 3). Assemblages of a cocrystallized orthorhombic mineral and rutile may represent either primary coprecipitation under temperature conditions lower than those generating the homogeneous precursor of the exsolution-inducedintergrowths, or a product of recrystallization of the exsolved aggregates.Low- s.6 90,8 9l.O 9!2 9o.0 *.2 90.4 p (") temperature conditions of cocrystallization also are sugested by the high Ti content of any orthorhombic phasethat did not undergo exsolution (e.9., some of Fig. 7. The p versus unit-cell volume diagram for the the LOT samples); metastable crystallization and wodginite-group minerals (after Ercit et al. I992a). preservation in a low-energy environment may have The ordering observed upon heating is proportional to been the cause. the length of vectors, which go from the data-points for natural minerals (tail-end of arrows) to tiose for heated Chemical composition samples (arrowheads).Thin lines: wodginite from the above study; heavy lines: onhorhombic phases from The orthorhombic phase and the exsolved rutile present the work, except A-25, which correspondsto type show the same compositional features as their analogs titanowodginite (from Ercit et al. 1992b). in rutile-hosted intergrowths (e.9., Sahama. 1978, eern! et al. 1981b,1986, unpubl. data of Pe). The J)U THE CANADIAN MINERALOGIST

TABIE 3. UNTT-CF'LL DIMENSiONS OF THE ORTHORHOMBIC PHASE K-l MSX NST/A NST/B KW BRD/C Nalual Smples a 4.752(1) 4;t35('L) 4384(t) 4;t'7'./(1) 4.109(1) 4;146(r) 4.728(L) 4.733(3) 4.1s5(3) 4.72s(r) 4;722(t) Ia] tl4.2s9l U4.2051 114.3s21 t14.3301 114.12'71 Lt423E) [4.1E4] u4.r99) t1426s) [r4.L7s) u4.166] b s.730(1) s;128(1) s.742(r) 5.?40(1) 5.712(1) s.72E(l) s;709(2) s.717(3) s.128(4) s.710(l) 5.703Q) 5.095(1) 5.120(1) s.088(1) 5.095(l) 5.107(1) 5.088(l) 5.113(l) s.133(s) s.13rQ) 5.130(1) s.r16(2) r38.73(5) 138.76(3)139.76(3) 139.68(5) 137.3s(3) r3E.32(4) 138.m(5) 138.36(9) t39.74(9) t3E.42(4) r37.76(s) M 14162l t4r6.491 1419.281 t4t9.0sl 1412.05) 14t4.961 1414.091 [4rs.0E] t4t9.22) t4ts26l 1413281 Heatedat 1000oC/16hous in air a t4.284(2) 14.261(2) 14.378(2) 14370(2) 14233(3) t4269(3) t42ss(3) 9.426(2) 9.418(2) 9.433(3) 9.43s(r2) b 5.733(1) s.731(2) s.742(1) s;74E(1,) s.720(1) 5.722(L) s.713(1) r 1.386(3) rr.382(2) r 1.384(3) 11.39s(6) c 5.V12(t) 5.012(1) 5.081(1) 5.074(1) 5.506(1) 5.060(r) s.065(r) s.081(1) 5.076(1) s.0'17(1) 5.07E(3) P' 90.72(t)" 90.70(1)o90.70(31 90.86(6r v 415.30(8)4r4.s3(9) 419.47(7)4t9.r0('t) 4r6.t2(8) 413.12(9)412.49(8) s4s2(2) 544.0Q) s4s2(2) 54s.9(2)

All data in A mits; dau in squile bracketslripled for dtecr compuison with orderedcell dimensionsof hearcdsmples; NST/A conespondsto composidom53-58, 60, 62: NSTiB43 io 49; BRD/C'73,j4,80; LOT/A 187;LOT/B 189,22tiLoT/c lgti TRT 5, 6, ?, 16.

crystal-chemical constraints regulating the chemical Structural state of natural phases composition of the fwo exsolved phasesare identical in both cases. The degree of cation disorder in the orthorhombic The orthorhombic phase occasionally incorporates phase tends to increase with increasing Ti content, but Ti to a degree closely approaching the stoichiometry the trend is disturbed by some highly disordered of wodginite ABC2O8or, more specifically, that of Ti-poor samples and the highly ordered Ti-rich ideal riranowodginite,(Mn,Fe)(Ti>Sn,Ta)(Ta,Nb)2Og minerals of Sveshnikova et al. (1965). Metastablecrys- (Ercit. et al. 1992b). For example, composition tallization of disordered and composirionally complex TRT 7 can be interpreted as (Mn2.65Fe2+,.osCao.rr)x.r,orthorhombic minerals can be expected to generate (Ti2.aoSno.e7Tao.orSca.17)2a.s,(Ta".0 rNb2.00)>8.0r O3z.oo. Many disordered phases, which would undergo variable compositions analyzed,in the TRT and LOT samples degrees of ordering in the solid state, commonly in approach this type offormula (Table 3), characterized conjunction with exsolution of the main substituents by (M,Ta)/Fe ,Mn) > 2 typical of the wodginire group or with recrystallization. @rcit et al. 1992a).However, in most compositions,the The rutile phase has the monorutile strucfure in (Mn,Fe) population of the A site would be excessive,and accordancewith its complex and variable composition, the excess could not be incorporated into the B site of which precludes long-range ordering into -like the ordered wodginite structure, despite the deficiency or other superstructures. in Ti and Sn. The rutile phaseshows a broad range of substitution Responseofthe orthorhombic phase to heating by the (Fe>>Mn)z*(Nb,Ta)2Ou and Fer*(M,Ta)Oo components, with widely variable Nb/Ia values. The For most samples of the orthorhombic phase with proportions of TiO2 and the above components are low to moderateTi content,heating produces the ordered randomly non-integral, and they do not approximate columbite structure. Only a minority of the samples any potential formulas with simple stoichiometry. examined develop the monoclinic structure of As shown in Figure 5, the extent of mutual wodginite. Which factors favor ordering to the solid-solution residual in the two exsolved phasesis wodginite structure? highly variable but positively correlated,with a distinct (1) The Ti content of the samples that adopt the tendency for rutile to retain a higher percentageof the wodginite pattern of order is among the highest orthorhombic component than the TiO, content of encounteredin our study ( 1.56to 4.21 Ti, 2.02 to 4.7 | the associated orthorhombic phase. The crest of a Ti + Sn atoms per 32 atoms of oxygen), as comparedto potential solvus between the orthorhombic phase the specimensthat order to columbite - tantalite (0.28 and rutile is evidently shifted off-center, closer to the to 1.45Ti, 0.28 to 1.58 Ti + Sn atoms per 32 atoms of (Fe,Mn) - (M,Ta) sideline. This again agreeswitl rhe oxygen). This is in considerablecontrast to stannian observationson niobian rutile with exsolved titanian ixiolite, which orders to wodginite for Sn contents as columbite - (dernf (Sn tantalirc et al. l98lb,dernf & Ercit low as 1.40 + Ti !.82) atomsper 32 atomsof oxygen 198s,1989). (unpubl. data of PC), and to the Sn-based natural TIIANIAN DqOLITE AND TITANIAN COLUMBITE -TANTALITE 559 wodginite sen.sustricto, which has Sn and (Sn + Ti) as all contain (TpNb) > 8 and (M,Ta)/(Fe,Mn) > 2. low as 1.06 atoms per 32 atoms of oxygen (Ercit et aI. However, a better statistical base is required to verify 1992a\. the role of this factor. (2) The rather substantial presence of Sn may play a role, as Sn+*does not fit the orthorhombic columbite Irnplications fo r mineral systematics structure as easily as Tia. Orthorhombic minerals may allocate their moderate Ti contents on ordering in the As stated in the Introduction, stannian and titanian ratio of the most common mechanism of substitution, ixiolite are defined by high contents of these respective Tia 3(Fe,Mn)z*_r(M,Ta)5*_20i.e., one third into the A metals and by ordering to monoclinic wodginite on site and two thirds into the B site. As shown above in heating, in contrast to disordered stannian and titanian (l), Sn#readily segregatesinto its own site, rendering columbite - tantalite, which orders to an orthorhombic the structure monoclinic, at distinctly lower formula supercell. It is evident from the preceding sectionsthat contentsthan doesTi. As shown in Tables 3 and 4, most true Ti-dominant ixiolite seems to be an exception of the samples that adopted the monoclinic structure rather than a rule among the Ti-enriched orthorhombic upon heating contain substantialSn, and even those phases, and titanian columbite - tantalite is quite that are Sn-poor in the two-dimensional surfaces of widespread. As in the case of the classic Sn-based polished sections may be Sn-richer in the bulk of their ixiolite, chemical composition does not seem to be at volumes used for XRD study. Considerable variation of present a reliable indicator of the mineral species,i.e., the Ti/Sn value was found in individual grains closely disolde,rsdtitanian columbite - tantalite versus titanian related to the LOT samples (Ferreira 1984). It should ixiolite; path of ordering upon heating is the only be noted in this respect that the type titanowodginite unambiguous criterion for identification. and all other compositions so far analyzed @rcit et al. L992a,b) also contain appreciable Sn. CoNcr-usroNs (3) The path of ordering could be predetermined by the presence of structural nuclei with short-range (1) Titanium-enriched membersof the orthorhombic order in the natural minerals. Such a possibility must be columbite family of minerals typically contain exsolved considered all the more becausethe monoclinic-ordering niobian-tantalian rutile, whereas homogeneousphases samples happen to be restricted to two closely related are much less common. Some homogeneousphases are bodies, the Tanco and Lower Tanco aggregated with a coprecipitated rutile phase, some deposits (Table 1). Conditions favorable for incipient occur in a general association, but not in contact with wodginite-type order in the orthorhombic (or pseudo- rutile. Rutile-absent occurrencesare extremely rare. orthorhombic?) natural phasescould have been realized (2) The bulk of the exsolved or coprecipitated in such a restricted environment but absent at other orthorhombic phase + rutile pairs is the Fe,Mn,M, localities. Ta-rich counterpart of the Ti-dominant pairs of niobian (4) The dominance of Ta over Nb, and Mn over Fe, rutile + exsolved titanian columbite. In both assem- may be significant in developing the wodginite blages, the rutile phaseis enriched in Fe2+,Ta, Fe+ and structure on heating the Ti-rich orthorhombic phases. Sn, whereasthe orthorhombic phasefavors Mn, Nb, Sc, The TRT and LOT samples are the only ones with both W andZr. Tal(Ta + Nb) and Mn/(Mn + Fe) greater than 0.50 (3) The orthorhombic phase has in most cases the (Table 3, Fig. 4). This is a feature of virtually all disordered structure typical of ixiolite and disordered compositions of the classic Sn-basedwodginite and columbite - tantalite. The degree of disorder tends to stannianixiolite tdernf & Ercit (1989, Figs. 4 and 9;, increase with increasing Ti content. On heatingomost Ercit et al. (l992a,Table2), and deposited datal. It is samples of the orthorhombic phase develop the ordered significant in this respect that Ercit et aI. (1992a) columbite superstructure, with Ti probably stoichio- successfully synthesized titanowodginite MnTiTarOr, metrically distributed between the A (Fe,Mn) and but phases with the wodginite structure were not B (M,Ta)z cation sites. In contrast to these titanian generated in any previous or present srudies of the varieties of true columbite - tantalite, some specimens FeMrOu - TiO, and FeTa2O6- TiO2 systems (cf Wise are transformed into the ordered monoclinic structure et al. 1998). of wodginite, which identifies them as titanian ixiolite, (5) Last but far from least, the presenceof "excess" tle disordered counterpart of titanowodginite. Ta over and above the stoichiometry of tantalite, (4) The development of the wodginite structure on (Mn,Fe)Ta2O6,may condition ordering to the wodginite heating is facilitated by high Ti contents and by domi- strucfure. Wodginite A4B4C8O32routinely contains nance of Mn and Ta over Fe and M, respectively.Both (Ta>Nb) > 8 on the basis of 32 atoms of oxygen, with these features are in agreement with the characteristics dl Nb in the C site and the "excess'oTa incorporated at of natural titanowodginite and with the Mn,Ta-based the B site (Ercit et al. 1992a).It is noteworthy that the composition of the only titanowodginite synthesized compositionst6aad7 ffiT) and 187,189,191 (LOT), to date. However, "programming" of the path of which order upon heating to the monoclinic structure, ordering by the presenceof wodginite-type or ordered s60 columbite-type nuclei may play a role, as may the oxide minerals in the Greer Lake pegmatitic granite and presence of appreciable Sn. Also, "excess" Ta in its pegmatite aureole, southeastern Manitoba. Am. Min- the disordered phase, over and above the (Fe,Mn) eral. 7I, 5Ol-517. (Nb,Ta), stoichiometry of columbite, may turn out to be HAwTHoRNE, F.C., LAFLAMMc, J.H.G. & significant. HwrHoruc, J.R. (1979): Stibiobetafite, a new member of (5) Further heating and XRD experiments are the pyrochlore group from VEZn6, Czechoslovakia. Can. required on natural samples of homogeneous Tirich Mineral. 17. 583-588. orthorhombic phases with negligible Sn content, and the synthesis of titanowodginite with substantialFe & NEMEc, D. (1995): Pristine vs. contaminated and Nb contents should be attempted, to verify the trends in Nb, Ta-oxide minerals of the Jihlava pegmatite potential controls given in (4). Also, HRTEM work is district, Czech Republic. Mineral. Petrol. 55, ll,7-129. desirable on phases natural orthorhombic to check on Paw, 8.J., HAwTHoRNE,F.C. & CHApMAN,R. the possible presenceof ordered nuclei, and on the (1981b): A niobian rutile - disordered columbite details of the exsolution history of individual samples intergtowth from the Huron Claim pegmatite, soutleastern (cl staringite: Groatet al. 1994). Crystal structures of Manitoba. Can. Mine ral. 19, 541-548. the heat-induced ordered orthorhombic phasesshould be refined to verify the distribution of Ti. TRUEMAN,D.L.,Zrr;gtxn, D.V., Gonp, B.E. & PAUL,B.J. (1981a):The Cat Lake - Winnipeg River and the Wekusko Lake pegmatite fields, Manitoba. Manitoba ACKNOWLEDGEMENTS Dep. Energy and Mines, Mineral. Res.Div., Econ. Geol. Rep. ER80-1. This study was supported by NSERC Research, Equipment and Major Installation Grants, DEMR - & TuRNocK,A.C. 1971:- minerals ResearchAgreements and logistical support from the from granitic at Greer Lake, southeastern Tantalum Mining Corporation of Canada, Ltd. to Pe, Manitoba. Can. Mineral. lO, 755-772. and by NSERC Equipment Grants to F.C. Hawthorne. ERcIr, T.S. (1986): The Simpsonite Paragenesis; the Crystal NSERC and University of Manitoba Scholarships to Chemistry and Geochemistry of Extreme Ta Fractiorntion TSE and MAW, whose main experimental contribution Ph.D. thesis, Univ. of Manitoba, Winnipeg, Manitoba. to this paper dates from their graduate studies, also are appreciated.The authors acknowledge the constructive CBruY, P. & HAwTHoRNE, F.C. (1992b) The reviews by L.A. Groat, M. Raudsepp and R.F. Martin, wodginite group. III. Classification and new species.Can which improved the original manuscript. Mineral.30, 633-638.

RereRENces & MCCAMMoN. C.A. (1992a): The wodginite group. tr. Crystal cheristry. Can. Arerrvmt, D.E.& Evers,H.T., Jn. (1973): Job 9214: indexing Mineral.30,613-631. and least-squaresrefinement of powder diffraction data.U.S. Geol. Surv., Comput. Contrib.20 (NTIS Doc. WrsB,M.A. & drnNf, P. (1995): Compositional PB2-16188). and structural systematics of the columbite group. Am. Mineral.80. 613-619. dennf, P., Cucu, F. & Povoruna, P. (1964): Review of ilmenorutile- striiveriteminerals. Neues Jahrb. Mineral.. Frnnrrn.+, K.J. (1984): The Mineralogy and Geochemistry of Abh. tOt, r42-r72. the Lower Tanco Pegmatite, Bemic lake, Manitoba, Canada. M.Sc. thesis, Univ. of Manitoba, Winnipeg, CHAIMAN,R., GOp,R., NTEDERMAvR,G. & WrsB, Manitoba. M.A. (1989): Exsolution intergrowthsof titanian ferrocolumbiteand niobian rutile from the Weinebene GoRzrDvsKAyA,S.A., SDoRENKo,S.A. & GNsnunc, A.I. spodurnenepegmatites, Carinthia, Austria. Mineral. (1974): Titano+antalo-niobates. Nedra, Moscow, Russia Petrol.4O. 197 -206. (in Russ.).

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